Thermocompression bonder



July 26, 1966 R. J. com] THERMOCOMPRESSION BONDER 6 Sheets-Sheet 1 Filed May 9, 1965 INVENTOR RONALD J CONTI Qfi ATTORNEY July 26, 1966 R. J. CONTl 3, ,0

THERMOCOMPRESSION BONDER Filed May 9, 1963 6 Sheets-Sheet 2 FIG. 2c

INVENTOR. RONALD J. CONTI ATTORNEY July 26, 1966 k. J. CONTI 3,263,057

THERMOCOMPRES S ION BONDER Filed May 9, 1963 6 Sheets-Sheet 5 sWlTCH INVENTOR.

RONALD J. CONTI J (in.

ATTORNEY United States Patent Office 3,263,057 Patented July 26, 1966 3,263,057 THERMOCOMPRESSION BONDER Ronald J. Conti, Anaheim, Calif., assignor to North American Aviation, Inc.

Filed May 9, 1963, Ser. No. 279,233 6 Claims. (Cl. 219-78) This invention relates to .a device for thermocompression bonding of metallic elements and more specifically to a device for thermocompression bonding of metallic elements, semiconductor elements and combinations thereof using balanced beam loading and joining tip supplied energy.

Thermooom-pression bonders of the type described herein are expected to find wide application Where size limitations preclude the use of filler and/ or fiuxing materials as in soldering. Microminiaturization and accompanying innovations in electronic art render processes such as resistance welding, which were suited for joining low conductivity metals and relatively larger electrical elements, less suited for applications which involve smaller, higher conductivity materials such as gold, platinum, and silver. It is because of continuing electronic microminiaturization that thermocompression bonders, with excellent heating efficiency characteristics and small component welding characteristics, have been used increasingly. Use of boudens in certain precision work has been limited, however, due to such limitations as the necessity for preheating a substrate to which an element is to be joined; also, many thermocompression bonders require the use of protective atmospheres to achieve satisfactory bonding. Substrate pre-heating, in the case of a circuit board replete with microcomponents, may cause damage or contribute to a shortened system life. Although the thermocompression bonding art can be said to represent an improvement over devices previously used for joining elements, efiiciency of present art thermocompression bonders is often too low for successful utilization with selected elements and substrate materials. Commercially produced thermocompression bonders utilizing moving tips are prone to have tip play or lack tip rigidity during bonding. Sometimes poor joining tip design causes an undesirable bonding, a sticking, of the joining tip to an element instead of bonding an element to another element.

In the device of this invention; bonding of metallic to metallics, metallics to semiconductor material, and various design embodiments thereof is achieved by the use of a new and improved joining tip means and balanced beam loading in conjunction with other means which together form a thermocompression bonder.

The thermocompression bonding system is comprised of joining tip means having various configurations, a lowfriction balanced beam loading means for applying a precise force at the selected juncture of the elements to be joined. The force may be varied to achieve greater bonding efiicien-cy depending on size, element configuration, and tip configuration. Power supply means furnish electrical energy which is converted into heat energy at the tip means. Other means such as bearing means for supporting the work elements, pyrometer means for measuring and controlling tip means temperature, indicator means for timing a particular bonding operation, microscopic means for placing the work, which is often too small to be accurately placed by use of the naked eye and for observing the operation, cooperate with the other means to form a thermocompression bonder which is an advancement in the state of the art of thermocompression bonders.

It is therefore an object of this invention to provide a highly eflicient system for bonding electronic component lead wires to metallic films by diffusion bonding.

It is another object of this invention to provide a highly eificient system for diifusion bonding of inner wire connections in transistors and other semiconductor devices.

It is another object of this invention to diffusion bond inter-connections in integrated circuit packages.

It is still another object of this invention to efficiently diffusion bond small diameter wires to ribbons to a base material.

It is still another object of this invention to provide a thermocompression bonder for joining components to metallic films and microminiaturized circuit packages by supplying all energy from the joining tip and without heating of the substrate material, in this case.

It is still another object of this invention to provide a thermocompression bonder for difiusion bonding of lead wires to semiconductor materials for forming transistors, diodes, and other semiconductor devices by supplying all energy from the joining tip and without heating of the semiconductor material, in this case.

Still another object of this invention is to provide a precise method of bonding force application for improving reliability and efiiciency of bonded joints.

It is still another object of this invention to provide a thermocompression bonder for diffusion bonding of lead wires to base materials in an air atmosphere (without the requirement to use any shielding gas).

Still another object of this invention is to provide a thermocompression bonder having low friction balanced beam loading means for improving reliability and efiien- .cy of bonded microminiature' elements.

A still further object of this invention is to provide a method of combining the principles of heat transfer and good design in order to achieve a device having very high efiiciency.

A still further object of this invention is to provide a process for achieving thermocompression bonding which is improved in efliciency over present art devices.

A still further object of this invention is to provide .a thermocompression bonder which eliminates tip play by use of a low friction balanced beam loading system as opposed to tip loading from above the tip, coupled with a moving tip.

It is still another object of this invention to provide joining tip means wherein the outer perimeter of the joining tip is heated to practically eliminate joining tip temperature drop due to heat absorption by the joint during bonding. It is still another object of this invention to provide joining tip means comprised of a first and a second electrode occupying a fixed spatial and insulated relationship with respect to each other.

It is still a further object of this invention to provide a joining tip'means comprised of a first and sec-0nd electrode having insulated and adjustable electrodes between which heat energy passes during bonding.

These and other objects of the invention will become apparent from the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is an illustration of one embodiment of a thermocompression bonder;

FIG. 2 is an illustration of several species of joining tips;

FIG. 3 is an illustration of thermocompression bonding using one species of joining tips;

FIG. 4 is an illustration of a multiple joining tip fixture;

FIG. 5 is an illustration of a thermocompression bonder utilizing servo-controlling means for applying and removing tip loading; and

FIGS. 6a, 6b and 6c are tables of data showing selected examples of tip material, temperature, loading, element composition, and substrate material.

Referring now to FIG. 1 wherein joining tip means 1 is connected to gear housing means 2 and rack assembly means 3. Gear housing 2 is supported by arm 4 which is secured to base portion 5 by mounting angles 6. Gear housing 2 includes joining tip height adjustment means 8 so that tip means of varying lengths and sizesmay be used without extensive modifications to the system. Joining tip height adjustment means 8 is locked in positionso that joining; tip means 1- is held securely during a bonding operation. Cable. means 7 supplies heat energy to tip means 1 from power supply means- 9 which, depending on the species of tip means being used, may be a standard resistance power supply, or low-voltage high amperage alternating current signal source, or other suitable power source. Cable means 39 connects power supply means 9 tmpyrometer means 10 so that tip 1 temperature is controllable. Cable means 40'connects tip 1 to pyrometer 10 by meansv of a thermocouple forcontinuous monitoring and control of. the temperature in conjunction with controlled? energy supplied-by power supply means 9. Use of a pyrometer is not required when bonding with the split electrode joining means described herein, in which case timermeans 26 is connected topower supply means 9 by cable means 44. During bonding the timer functions to shutoff bonding current after preset intervals. In addition the timer turns on current when actuated by microswitch means 17. Upper bearing stage assembly means 11 rests on sub-stage assembly means 12. Sub-stage assembly means 12 rides in stage support means 13. The upper bearing stage means serves as a support for elements to be bonded. The sub-stage assembly acts as a shaft to which the support is movably attached. Beam assembly means 14 which is comprised of two parallel bars 27' and 28 held together at measured intervals by rod members 29 through 34. Beam assembly means supports sub-stage assembly means 12 by roller bearing installation 35 at each side of sub-stage assembly means 12. In-addition beam assembly means 14 is bearing supported by beam support means 15. Bearing means 36 provides low friction support for beam 14 on support means which acts as a fulcrum element for the beam and support. Balance adjust 37 is a threaded weight which is adjustable on threaded rod- 38' and connected to bracket means 29 forming an end portion of beam 14. Bearing stage assembly means 11 is rotatable in 360 in ahorizontal plane and is'mob'ilein both the'X and Y planes. Bearing assembly means 11 is restrained by an oil film and a machined ridge to prevent separation from sub-stage assembly=12. The natural attractive force of the oil film does not require any further mechanical connection. Beam support means 15 is connected to base portion means 5 by bolting or other connection means. Weighing means 1'6'for example, a square shaft block of lead having connection hooks, is suspended from beam assembly means 14 by shafts provided therein. Weighing means 16 applies a torque about the balanced axis of beam assembly means 14 so that the upper stage assembly. means 11 exerts a force against joining tip means 1 in a practical application. Forv example, inan operation whereby a component lead is being bonded to a metallic film circuitry attached to a substrate, the joining tip applies a torque'about the balanced axis, beam support means 15, equivalent to the torque applied by weighing means 16 suspended from shaft support means 14. The torque can be, precisely calculated to provide bonding loa'dand may be changed depending on the force required. Accuracy and precision are greatly increased by useof the balanced beam or dead weight loading system described herein. Before weighing means 16 is connected to any one of rod like'members30 through 34, the beam assembly means is balanced; The elements to be joined (or an equivalent weight) are placed on stage assembly means shown in FIG. 1 as substrate assembly 41 and the balance adjust means 37 is adjusted until the beam means is exactly level. Post 42 and pointer 43 indicate when a balance is achieved. Because of bearing installation and bearing means 36 providing a very low friction support, when weight 16 is connected to beam means 14, its torque is substantially directly applied to load tip means 1 in a bonding operation. Weight 16 can be made extremely accurate so that tip loading can be made very accurate for highly precision work involving microcomponents and elements. In addition, one weight may be used to apply other required forces by merely moving it to another position on the beam assembly means. Microswitch means 17 is pivot connected at the end of beam assembly means 14 and includes axial support means 18. Cam assembly means 19 is attached to base portion means 5 and is comprised of lever means 20 and cam means 21. Cam means 21 and microswitch means 17 comprise cam follower means 22. The bender also includes timer means 26 for controlling the bonding cycle and includes means for being actuated by microswitch means 17. Microscope means 24 is connected to base portion means 5.

Lever means 20 may be manually operated to apply tip pressure (loading) to elements placed on bearing means 11; or a servo system comprised of motor means 25 connected by shaft means to cam means 21 and time indicating means 26 may be utilized to apply and remove tip loading after a preset period of time sufiicient for bonding has elapsed. A block diagram embodiment is illustrated in FIG. 5 wherein motor 25 may be switched on by switch means 44 and slowly raise bearing means 11 until tip means 1 is in contact with the elements to be joined, at which time microswitch means 17 turns on to actuate timing means 26 and cut off motor means 25. After a preset duration has elapsed, as set in timer 26, the timer indicates bonding is complete and actuates motor means 25 to initiate continued rotation of cam means 21 until the tip loading is removed. Servo motors of a type which may be employed with minimum design changes, obvious to persons skilled in the art, are described in Servo Mechanism Analysis by Thayler & Brown, on pages 385-392. Timer 26 may be incorporated in-motor means 25 so that microswitching means 17 would be connected only to motor means 25 for actuating motor means on and oil as required.

Referring now to FIG. 2 wherein are illustrated several species or" joining tip means 1, for example tips 2, 3, and 4 shown are constant temperature resistance heated tips. Tip 3 is better utilized in making interconnections involving smaller elements whereas tip 2 can be better utilized with larger elements. Tip 4 design simplifies application involving semiconductors. Basic geometry at the base portions 5, 6, and 7 of the tip means is similar for tips 2', 3, and 4. Holes 8, 9, and 10 may be included for the insertion of thermocouple for monitoring and controlling tip temperature. The outer diameter of base portions 5, 6, and 7 is selected relative to the particular peripheral heating source being used. Larger diameter tip bodies or base portions may fit into commercially available heater means, whereas small tip bodies may require special heater fabrication. For example, a combined heater-tip may be constructed by winding approximately, twenty-five turns of nichrome wire around a beryllium oxide ceramic tip body of approximately 0.040 inch in diameter. After winding, it may be coated with a compound such as sauereisen number one. Then if desired or necessary for more thermal energy, an additional layer of wire may be wound about the previous combination and again coated. In addition, heater means may be fabricated by winding for example thirty-twoturns of 0.0l6 inch diameter nichrome wire about a 1.5 inches length of cylindrical glass tubing into which a tip is inserted and secured. Use of a glass tube holder will permit rapid interchangeability of tips. The area in the vicinity of termination of tips 2, 3, and 4 are exposed after insertion into the heater means. The mass of the heating means surrounds the base portions 5, 6, and 7 in all cases so that total heat content of a tip is high prior to bonding. Peripheral heating eliminates the problem of tip temperature drop during bonding due to heat absorption at the joint.

Tips shown in FIGS. 2a, 2b, and 20 may be fabricated of copper-chrome alloy which is chrome-plated; or of beryllium oxide mounted in copper-chrome alloy which is chrome-plated. Alumina, beryllia and similar materials and alloys will function but are not as efficient as beryllium oxide. approximate efficiency as copper-chrome. A beryllium oxide ceramic tip may be used as a stable (non-oxidizing) tip. FIGS. 2a, 2b, and 2c show end views of tips 2, 3, and 4. FIGS. 2d and 2d are illustrations of a tip having a first and second electrode occupying a fixed spatial and insulated relationship with respect to each other. Electrode 11 and electrode 12 may be comprised of materials having good oxidation resistance, high electrical resistivity (relative to material being bonded), poor thermal conductivity (low joint heat depletion vertically through the tip away from the joint). A suitable electrode combination is nichrome and nichrome; or other high resistivity material. Suitable dielectrics are mica, alumina, and materials of similar properties. The material being joined acts as a current bridge between the separated electrodes. By use of the split electrode joining tip, the maximum temperature zone is in the material being joined, for example the lead wire. Temperatures at the joint with the split electrode tip are very high. It is possible to bond materials such as component lead wires to very thick metal sheet circuit films, for example films having thicknesses of approximately 0.003 inch. Bonding efficiency is normally decreased when thick films are used in the bonding operation where the tip is heated. The split electrode tip may be connected to a standard resistance power supply. It may be desirable in some instances to include means for adjusting spacing between electrode 11 and electrode 12 so that maximum and minimum spacing may be achieved. An air gap between the electrodes will provide sufiicient insulation in lieu of other insulating materials or means. Conductors 13 and 14 conduct energy from a power supply through elements being joined.

Simultaneously joints such as the joining of a plurality of lead wires to form a fabricated electronic circuit may be achieved by the multiple tip joining fixture 15 shown in FIG. 4. The multiple tips may be any one of the types illustrated in FIGS. 2a, 2b, 2c, and 2d and may be any eificient number. It is preferable, so that temperature diiferential between joining tips is minimized, to use identically designed tips for the fixture. -Since lead wires and other materials to be joined may not lie in the same exact plane, compensation is provided by spring loading means 16, 17, 18, and 19. By use of the spring loading means varied vertical deflection can occur between tips during bonding so that damage to either the substrate or materials being joined or the tip would not be incurred. Excess deflection is limited by preloading the individual springs by sleeve means 20, 21, 22, and 23 for each spring means, to some force less than required for bonding. Thus deflection of one does not begin until preset minimum is exceeded, and the total force requires that some deflection occur in each of the tips. The feature of low friction precise beam loading is still present, however, the spring loaded tips primary function are to provide deflection-loadin'g accuracy is still the function of the beam assembly. Fixture 15 uses split electrode tips 'having energy supplied through conductors connectedto each electrode. The tips are connected to shafts by sleeve connectors 24, 25, 26, and 27 so that individual tips may be replaced or disconnected. Multiple tip fixture 15 represents only one method of achieving tip multiplicity, peripherally heated tips could be used or combinations of the tips shown could be used. Use of XY micrometer type support fixtures for tips could provide location latitude for multiple tips.

The thermocrompression bonder may be used to bond lead wires comprised of materials such as gold, silver,

Beryllium metal will function with the copper, gold plated copper, gold plated nickel or kovar, gold alloys, and combinations thereof to plane surfaces or film materials such as gold, pure and alloyed with platinum, tin, silver; also silver, nickel, tantalum, tin and alloys, aluminum, silver, copper, gold plated films. A practical maximum diameter wire size for use as a lead wire is 0.005 inch. Lead wires may be component lead wires for resistors, transistors, diodes, capacitors, connectors, inductors, and other such components, or they may be wires alone for connecting elements in other configurations and uses. In other applications the film or surface elements are attached to a substrate comprised of materials such as glass, quartz, micas, alumina, and other rigid materials to form a circuit board. In addition to the above examples, semiconductor materials such as silicone and germanium may be bonded to circuitry attached to a substrate, elements may be bonded to semiconductor materials for forming diodes and other semiconductor devices, leads may be bonded to fabricated electronic circuit blocks of the micro-min variety, and semiconductor materials may be bonded directly to other semiconductor materials. Whenever semiconductor materials are bonded directly to circuitry, component leads are not required. A substrate having a size of 1.5" by 0.075" by 0.0060" is one example of a microminiaturized substrate having film circuitry attached thereto to which other elements may be bonded using the thermocompression bonder described herein. The preceding list of examples is not intended to be exhaustive but serves to provide illustrations of types of materials which may be thermocompression bonded.

FIG. 3, which is an illustration of thermocompression bonding using one species of the several speciesof joining tips discussed, is used in connection with a description of a bonding operation. A tip such as any one of the tips illustrated in FIG. 2 or a tip fixture as described in connection with FIG. 4, is connected to the rack assembly. Elements to be joined, for example, a component lead wire 45 to metallic circuitry 46 attached to a substrate 47 is placed on hearing stage means 11 and is positioned under joining tip means 1 by the use of microscope means 24. The joining tip is adjusted by means of joining head height adjustment 8, weight 16 is properly connected to beam assembly means 14 so that the desired tip pressure (loading) is achieved. Power supply means 9 is adjusted in connection with pyrometer means 10 for heating the tip to a desired temperature. Indicator means 26 is set at a time predetermined to be sufficient to bond the particular elements involved. Cam lever means 21 is rotated either by manual means, or by switching and servomotor means so that the beam assembly means 14 raises the bearing stage 11, having the elements to be joined thereon, so that the joining tip means 1 comes in contact with the elements to be joined. For example, referring to FIG. 3, the joining tip means 1 loads against lead'wire 45 so that lead wire 45 and the metallic film 46 are forced together. Metallic film 46 is attached to substrate 47. Dilfusion bonding occurs at bonding area 48 within a time depending on the particular materials being bonded. After bonding is completed cam 21 rotates to lower stage means 11 and to remove the tip pressure (loading) from the juncture.

If the element surface to be joined is corroded or otherwise unclean it may be necessary to abrase the surface, for example, by use of a pencil erasure and alcohol rinse. Copper oxides may be removed by dipping in a solution for example comprised of 10% HCL just prior to bonding followed by an alcohol and a water rinse. Such cleaning requirements are only necessary where clean metal to metal contact is not obtained.

Examples of early test results of bonding time, bonding load (Weight 16), bonding temperature for joints of certain metallic elements for various tip materials are tabulated in FIGS. 6a, 6b, and 6c. Bonding time can be adjusted as low as 0.1 second depending on the terminal joint size and other variables. It can be seen by comparing the data contained therein that the various parameters are interrelated; for example as bonding load is increased the bonding area is increased and greater surface contact between the elements is made so that less time or temperature isrequired to achieve bonding. Diffusion is a function of bond temperature and time, with greater diffusion occurring at the higher values thereof.

SUMMARY This invention relates to a. thermocompression bonder having new and improved joining tip means for achieving diffusion bonding. or interface reaction between metallic elements, metallic and semiconductor elements. Joining tip means, applied to the elements to be joined by balanced beam loading means which provides accurate loading, in one species is heated peripherally by heating unit into which the tip fits. Bearing means selectively located provide relatively low friction loading. In another species the heating unit iscomprised of a transformer providing selective low voltage and high current across first and second electrodes of a joining tip whenever the electrodes are pressed into contact with the elements to be joined- With either species of tip design, the total bonder. design described herein is of such high efficiency so that bonding in an air atmosphere is achieved; Necessity for using a gas shield is eliminated;

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. In combination:

means for simultaneously diffusion bonding a plurality of elements, comprising a plurality of spring loaded joining means each having a first and a second electrode means in fixed spatial and insulated relationship with respect to each other and wherein each of said joiningtips occupies a fixed spatial relationship with respect to each other;

means forming a beam and a fulcrum, said beam being rotatable, relatively friction free about said fulcrum with the rotatable axis being. parallel to the horizontal plane;

means for supporting elements tobe diffusion bonded,

said means being horizontally rotatable and mobile in a vertical and horizontal direction; means connected to said'means for supporting for compressing said elements with said stationary joining tip means, said: support means being mounted for relatively frictionless. motion along said beam;

means for supplying electrical power to said joining tip means.

2. In combination:

means for simultaneously diffusion bonding a plurality of elements, comprising a plurality of spring loaded stationary joining tip means each having a firstand a second electrode means occupying a fixed spatial and electrically insulated relationship-with respect to each other and wherein-each of said plurality of joi-ningtip-means occupies an adjustable spatial relationship with respect to each other;

means forming a beam and a fulcrum, said beam being rotatable, relatively frictionfree about said fulcrum with the rotatable. axi-sbeingjparallel to-the horizontal plane;

means for supportingelements to be diffusion bonded,

8. said means being horizontally rotatable and mobile in a vertical and horizontal direction; means connected to said means for supporting for compressing said elements with said stationary joining tip means,v said support means being mounted for relatively frictionless motion along said beam; means for supplying electrical power to said joining tip means. 3; A. process for thermocompression bonding electrical elements, comprising the steps of:

placing elements to be bonded on bearingmeans; applying stationary joining tip means-to said elements with a predetermined pressure at a desired joint.location, said joining tip means comprised of first and second electrode occupying a fixed spatial and insulated relationship with respect to each other and applying an electrical potential difference between said electrodes insufficient to cause fusion of said elementsexcept under said predetermined pressure. 4. A process for thermocompression bondingelements, comprising the steps of:

placing elements to be: bonded on a. support means; applying stationary joining tip means to said elements at a desired joint location, by balancedbeam loading means, said joining tip means comprised offirst and second electrode occupying an adjustable and insulated relationship withv respect to each other and subsequently applying an electrical potential difference to said electrodes insuflicient to cause diffusion except under the pressure provided by said beam loading means. 5. A process for thermocompression. bonding two or more elements, comprising the steps of:

placing elements to be bonded on support means; applying. stationary joining tip means to said elements at a desired joint location by balanced beam loading means with predetermined pressure, said joining tip means comprised: of abody port-ion extending within means for heating whereby said. joining. tip means is heated to a desired temperature. 6. The. compression diffusion-bonding process comprising the steps of:

juxtaposing elements tovbe diffusion bonded; compressing a selected portion of said elements for forming a compression diffusion bondingarea; locally heating only the compression diifusion-bonding area to the diffusion temperature, said local heating being achieved by an electric current flow through one of said-elements; and maintaining the local heating effect only for the time necessary to achieve a diffusion bond. at said compression diffusion-bonding area.

References, Cited by the Examiner UNITED STATES PATENTS 183,156 10/1876 Grove 177-252 X 934,538 9/1909 Johnson 219- 1,674,653 6/1928 Mann et al. 219238 1,743,755 1/1-930 Casella et a1 21985 X 2,394,822. 2/1946 Teplitz 21986 2,452,009 10/ 1948 Woodward 21986 2,794,899 6/ 1957 Plummer 21'9--85 3,050,617 8/1962 Lasch et al 21985 RICHARD M. WOOD, Primary'Examiner.

B. A. STEIN, Assistant Examiner; 

1. IN COMBINATION: MEANS FOR SIMULTANEOUSLY DIFFUSION BONDING A PLURALITY OF ELEMENTS, COMPRISING A PLURALITY OF SPRING LOADED JOINING MEANS EACH HAVING A FIRST AND A SECOND ELECTRODE MEANS IN FIXED SPATIAL AND INSULATED RELATIONSHIP WITH RESPECT TO EACH OTHER WHEREIN EACH OF SAID JOINING TIPS OCCUPIES A FIXED SPATIAL RELATIONSHIP WITH RESPECT TO EACH OTHER; MEANS FORMING A BEAM AND A FULCRUM, SAID BEAM BEING ROTATABLE, RELATIVELY FRICTION FREE ABOUT SAID FULCRUM WITH THE ROTATABLE AXIS BEING PARALLEL TO THE HORIZONTAL PLANE; 